The humans and the war reminds me of the greek persian war and humanities steriotipical tendencies. The persians passed through the greek city states like nothing, with their overwellming navy and infantries they attacked the greeks until arriving at the mainland of the greek state, home to places like athens and sparta. Then, you have the covenants utilizing their starships and pulverizing colonial planets (mind you attacking some far off colony is easier then attacking the heart of humanity, now they are basicly trashing earth in halo 2).

The big turn around the greek matter was that after a serious of victories in the mainland, the greeks pushed the persians off with a movie like comeback. Because they've said that some of the levels will actually be on earth and most would be on the covenant planets, would lead us to believe that humanity will have the same turn around that the greeks did with the persians.

When I saw the halo 2 movies and heared about the story, I'm surprised that the covenants would occupy earth with troops. They've glassed entire planets so that they didn't need confrontation with Infantries. I expect the whole covenant navy to be around earth, the simple idea of sending troops to earth would only lenghten the time of humanity's survival.

Why would the covenant want to send troops to earth, what is it that they want, if anything? maybe there's something that they are looking for?

The various mentions by Bungie staff that the fight will later be taken to the Covenant worlds do imply that the assault on Earth will be repelled eventually, or to some degree. But once again, how? Perhaps Humanity's destruction is not the primary goal, but only an obstacle on the way; even a tertiary objective? Might we find what the Covenant is looking for first...